Ubiquitin C-terminal hydrolase L1 (UCH-L1), identical to the neuron-specific protein gene product PGP9.5, is a member of the deubiquitinating enzyme family. The biochemical function of this enzyme is to release free and reusable ubiquitin from ubiquitinated proteins. Although there is evidence showing that the expression of UCH-L1 is associated with malignant features in cancer patients, the exact roles of this protein and molecular mechanism involved are not known. We have found that UCH-L1 was overexpressed in several drug resistant cancer cell lines and in poorly differentiated breast and colon carcinomas. We also observed that the cancer cells with high expression of UCH-L1 had lower protein abundance of p27, a cell cycle inhibitor, and possessed enhanced invasive ability. In this proposal, we describe studies designed to decipher the roles of UCH-L1 in cancer progression. Specifically, we plan to investigate whether UCH-L1 plays a role in determining the invasive/metastatic ability and drug sensitivity of cancer cells using UCH-L1 expression vector, RNA interference, and other molecular biology, cell biology, and pharmacology approaches. Through these studies, we expect to gain an understanding of the pathophysiologic and pharmacologic functions of UCH-L1 in cancer. Since ubiquitin system is present in almost all cells and has numerous protein targets, and deubiquitinating enzymes are part of this system, our studies of UCH-L1 may potentially be applicable not only to cancer but other diseases.